Internet entrepreneur Nicholas Merrill was working in his Manhattan office when an FBI agent in a trench coat arrived with an envelope.

It was fall 2004, and federal investigators were using new legal authority they had acquired after Sept. 11, 2001. Merrill ran a small Internet service provider with clients including IKEA, Mitsubishi and freelance journalists…

The agent handed Merrill a document called a National Security Letter, which demanded that he turn over detailed records on one of his customers. The letter wasn’t signed by a judge or prosecutor. It instructed him to tell no one.

Thanks to new laws and technologies, authorities track and eavesdrop on Americans as they never could before, hauling in billions of bank records, travel receipts and other information. In several cases, they have wiretapped conversations between lawyers and defendants, challenging the legal principle that attorney-client communication is inviolate.

Advocates say the expanded surveillance has helped eliminate vulnerabilities identified after the Sept. 11 attacks. Some critics, unconvinced, say the snooping undermines privacy and civil liberties and leads inevitably to abuse. They argue that the new systems have weakened security by burying investigators in irrelevant information…

A robust debate on the intelligence gathering has been impossible, for the simple reason that most of the activity is officially secret. In lawsuits alleging improper eavesdropping, the Justice Department has invoked state secrecy to prevent disclosure of classified information and systems.

In May, two members of the Senate Intelligence Committee said that Americans would be disturbed if they knew about some of the government’s data-gathering procedures. But Sens. Ron Wyden (D-Ore.) and Mark Udall (D-Colo.) said they were prohibited from revealing the facts…

The National Security Agency, which eavesdrops on foreign targets, once had to get a court-approved warrant to monitor a U.S. citizen’s communications over wires that traverse the United States. Now the agency is free to vacuum up communications by Americans and foreigners alike, as long as the target of the surveillance is a foreigner…
“I want the America back that I was taught about in school,” Merrill said. “The one where there’s checks and balances, and where one branch of government can’t do everything on its own.”